Method for the layered construction of models

ABSTRACT

Described is a procedure for layered composition of models, whereby at least a first material is applied to an assembly platform, followed by selective application of a second material layer; these two application steps are repeated until the required model is achieved and both materials form a solid structure in an appropriate mixture ratio. The first material comprises a moulding sand; the first and/or second material comprises a bonding agent encompassing a salt-crystal binder and/or protein binder.

This invention involves a procedure for layered composition of models.

State-of-the-art examples of this procedure include rapid-prototypingmanufacture of moulds and casting models without a use of tools.

For instance, DE 198 53 834 A1 describes a rapid-prototyping procedurefor creating cast models. In this case, an untreated particle materiallike quartz sand is applied in a thin layer to an assembly platform.After that, a spraying mechanism is used to finely distribute a bondingagent over the entire particle material. Selected areas of the resultingsurface are subsequently covered with hardener to reinforce the particlematerial wherever required. Repeating this whole process several timespermits a structure to be shaped individually from the bonded particlematerial. This structure is initially embedded in the encompassing,unbound particle material and can be removed later following completionof the assembly process.

For example, if this type of rapid-prototyping procedure makes use ofquartz sand as the particle material and furan resin as the bondingagent, sulphuric acid as the hardener makes it possible to create amould consisting of materials normally employed during mould productionand therefore familiar to experts.

The binder largely consists of furfuryl alcohol, nitrogen, water andfree formaldehyde. Concentrated sulphuric acid is normally used as theactivator.

A major disadvantage of moulds so manufactured are the environmentallyharmful constituents of the binder material. This results in complexhandling and recycling of such material, especially during actual mouldmanufacture, casting, de-moulding and disposal of the casting sand.

During casting, the organic binders are decomposed into gaseoussubstances. This gas formation can negatively affect the cast componentby allowing gases to permeate the metal and make the workpiece porous.This can notably worsen component quality.

In the case of such modelling, however, it is important for the binderto decompose into organic substances in order to permit subsequentcoring of the cast metal part.

Furthermore, the crack products arising during decomposition of thebinder are environmentally harmful and need to be disposed ofappropriately.

Demoulding of the cast part can require a great deal of energy, becausethe binder needs to be destroyed thermally before it can be removed fromthe cast part's cavities.

The sand employed as the particle material must be reconditionedthermally after casting, i.e. residual bonding agent must be combustedthrough intense heating of the sand. This, too, requires a lot of energyand also gives rise to environmentally harmful reaction products.Alternatively, the sand can be deposited on landfills, although thismeasure is not very environmentally compatible either.

In general, however, organic binders can be integrated very favourablyinto layering techniques. A multi-component resin is usually employedfor this purpose. In addition to the afore-mentioned option ofselectively applying two components in succession to the untreated sand,it is also possible to mix a reaction component into the sand andselectively dose a second component. Either this proves sufficient for asubsequent, selective combination of the sand particles, or additionaluse is required of thermal energy or a reactive gas during/after theassembly process.

U.S. Pat. No. 5,204,055 proposes manufacture of metal moulds via 3-Dpressing. Here, the basic material comprises aluminium oxide ceramicparticles, the bonding agent a colloidal silica suspension. Due to poorcoring properties, however, this material system is unsuitable for sandcasting, and exhibits notable discrepancies with respect to the usualsand casting characteristics.

Moulds so produced possess a certain green strength after assembly. Toachieve their final strength, the moulds must undergo thermal transfer.In this process, cracks can occur in the moulds, or the componentdimensions can change as a result of contraction.

In such cases, demoulding of the cores is performed in a specialsolution or by means of a water jet.

As experts will know, Z-Corp. uses a gypsum-based system formanufacturing moulds. One disadvantage of this type of material is thedivergence of its casting properties from those of sand casting; thereare no similarities here.

The invention under consideration is accordingly intended to provide aprocedure for layered composition of models—i.e. moulds and cores—makinguse of environmentally friendly bonding agents fulfilling specifiedcriteria for moulded material such as strength and stability. Moreover,the casting properties are to be similar to those of conventional sandcasting.

This is accomplished by a procedure for layered composition of models,whereby at least a first material is applied to an assembly platform,followed by selective application of a second material layer; these twoapplication steps are repeated until the required model is achieved andboth materials form a solid structure in an appropriate mixture ratio.The first material comprises a moulding sand; the first and/or secondmaterial comprises a bonding agent encompassing a salt-crystal binderand/or protein binder.

Salt-crystal or protein bonding agent used in a procedure for layeredcomposition of models is characterized, in particular, by itsenvironmental compatibility during production, mould handling, castingand disposal.

Furthermore, this type of procedure in conjunction with mouldingmaterials commonly used in foundries makes it possible to achieve modelswith very good material characteristics highly similar to thoseassociated with sand casting.

After casting, the resultant component can be demoulded very easilythrough immersion in water or shaking. This is permitted by the highwater solubility of salt-crystal binder.

In a preferred embodiment of the procedure forming part of thisinvention, the bonding agent is mixed into the first material.

The first material in the procedure forming part of this invention isexpediently a mixture comprising bonding agent and moulding sand.

This makes it possible to add a type of salt to the sand. Water is thenadded selectively to the resultant mixture. The salt dissolves in thewater to encompass the sand. After the sand has dried, the saltcrystallizes to bind the sand particles.

This material's behaviour during casting is absolutely neutral. Thesalt's melting temperature is notably higher than that of the metal,thus preventing a generation of gases during casting. However, themoulded material needs to dry optimally to prevent boiling retardation.For this purpose, conventional core-shooting techniques make use ofmicrowave radiation to dry the core. This would also be possible for theprocedure forming part of this invention. The model can furthermore bepurged with warm air.

After casting, the mould can be cored through immersion in water, whichdissolves the salt and related bond.

The sand can be re-used after casting. Yet another advantageous castingproperty here is the neutral odour compared with organic bonding agents.

In the procedure forming part of this invention, it is also possible forthe moulding sand to be coated with the bonding agent.

In a preferred embodiment of the procedure forming part of thisinvention, the bonding agent is mixed into the second material.

Good results are achieved if the first material comprises moulding sand,the second material a solvent.

If the solvent essentially comprises water, it is fully compatible withthe environment and very economical.

It is preferable for the second material to be applied by means ofdroplet generation.

In addition, it is possible for the second material to be applied bymeans of screen printing or spraying through a template.

In the procedure forming part of this invention, it is especiallypreferable for the solvent to be removed through drying after anappropriate reaction time has elapsed.

In the procedure forming part of this invention, the moulding sandpreferably comprises quartz sand, zircon sand, olivine sand and/orfireclay sand.

The bonding agent in the procedure forming part of this invention isbased preferably on magnesium sulphate, sodium polyphosphate and/orprotein.

The described procedure forming part of this invention has provenparticularly suitable for manufacturing components serving as moulds formetal casting.

Examples of preferred embodiments below are used to describe theinvention more closely.

A procedure for layered composition of foundry moulds according to thisinvention is described in the following.

In contemporary procedures, binder is mixed with basic mouldmaterial—usually comprising quartz sand—and applied in thin layers usinga coater to defined areas of a lowerable assembly platform. Acomputer-controlled pressure head applies activator to the porous sandlayer over the required cross-sectional areas of the component to begenerated. The final component is achieved successively by repeatingthis entire process, i.e. lowering the assembly platform by one layerthickness unit, applying a thin layer of sand and binder, andselectively dosing activator.

According to the invention, the bonding agent comprises a salt orprotein binder.

The binder can either be pre-mixed with the sand prior to layering, oradded as particle material to the sand. Moreover, it is possible toselectively apply the binder in a solution and/or dilution by means of adosage head in accordance with the cross-sectional profile which needsto be achieved.

The bonding agent can be employed in a variety of ways as part of thelayering process. For example, it can be mixed in the form of solidparticles with the sand. The resultant mixture is then applied in layersto an assembly field. After that, water or a solvent is applied by meansof a droplet generator (alternatively: screen printing, spray through atemplate) to the required cross-sectional areas of the component. Aftera short reaction time, the water is removed through drying (passivedrying, microwave radiation, heat radiation, warm air stream etc.). Theentire process is repeated by lowering the assembly platform andapplying a new layer.

It is also possible to remove the water from the bond after completionof the entire assembly process, although this poses a danger ofdiffusion which can disperse the component's geometry.

Another option is to coat the sand with the binder prior to the processand employ it as described above.

Yet another possibility is to apply a mixture of binder and water tountreated sand.

In all procedural variants, the resultant model is freed of surroundingmaterial after the assembly process, and the sand made available forrecycling.

It is important to dose the solvent in the exactly required quantities.On one hand, sufficient solvent should be dosed to bind the particlesmutually and with the underlying layer. On the other hand, the solventdosage should be kept within certain limits to prevent undesireddiffusion and resulting impairment to the model's outline and accuracy.

Particularly good results are achieved if the first material comprises amoulding sand like quartz sand and 1.8% by weight of LaempeKuhsBinder®(a binder made by Laempe). The second material dosed in the preferredembodiment exemplified at the start comprises 3% by weight of water.

GMBond protein binder from Hormel is particularly suitable for thefoundry applications associated with the procedure forming part of thisinvention.

Quartz sand is highly suitable as a basic material also when mixed withprotein binder.

1. A procedure for layered composition of models, comprising the stepsof: applying at least a first material layer that includes a mouldingsand and optionally a bonding agent that includes a salt-crystal binder,a protein binder, or both to an assembly platform, and applying a secondmaterial layer that includes a moulding sand and optionally a bondingagent that includes a salt-crystal binder, a protein binder, or both;and repeating these two application steps until the required model isachieved and both materials form a solid structure in an appropriatemixture ratio, the first material layer, the second material layer orboth comprises a bonding agent comprising a salt-crystal binder, aprotein binder, or both.
 2. A procedure according to claim 1, wherebythe bonding agent is mixed into the material of the first materiallayer.
 3. A procedure according to claim 1, whereby the first materiallayer includes a mixture comprising a solvent, the bonding agent andmoulding sand.
 4. A procedure according to claim 3, whereby the mouldingsand is coated with the bonding agent.
 5. A procedure according to claim1, whereby the bonding agent is mixed into the material of the secondmaterial layer.
 6. A procedure according to claim 1, whereby the firstmaterial layer comprises moulding sand and bonding agent, which isselectively contacted with a solvent.
 7. A procedure according to claim6, whereby the solevent essentially comprises water.
 8. A procedureaccording to claim 1, whereby the solvent is applied by means of dropletgeneration.
 9. A procedure according to claim 1, whereby the solvent isapplied by means of screen printing or spraying through a template. 10.A procedure according to claim 1, whereby the solvent is removed bydrying after an appropriate reaction time has elapsed.
 11. A procedureaccording to claim 1, whereby the moulding sand comprises quartz sand,zircon sand, olivine sand and/or fireclay sand.
 12. A procedureaccording to claim 1, whereby the bonding agent comprises magnesiumsulphate, sodium polyphosphate and/or proteins.
 13. (canceled)
 14. Theprocedure of claim 1, whereby the model is metal casting mould
 15. Aprocedure for layered composition of a metal casting mould, comprisingthe steps of: a) mixing solid particles of a bonding agent comprising asalt-crystal, a protein or a combination thereof, with a sand thatcomprises quartz sand, zircon sand, olivine sand, fireclay sand or acombination thereof, to form a bonding agent/sand admixture, b) applyinga thin layer of the bonding agent/sand admixture to and assembly fieldof an assembly platform; c) selectively applying a solvent to thebonding agent/sand admixture in required areas; d) lowering the assemblyplatform; and e) repeating at least steps (a)-(c) for applying anadditional layer.
 16. The procedure of claim 15, characterized in thatthe solvent is water and is applied in a sufficient dose so that it iscapable of bonding particles of sand within a layer to each other, tounderlying sand particles that may be present; and the moulding sandcomprises quartz sand zircon sand, olivine sand, fireclay sand or acombination thereof; and
 17. The procedure of claim 15, characterized inthat the sand includes a quartz sand and the bonding agent includes asalt-crystal binder.
 19. A metal casting mould including a plurality oflayers of a bonding agent/sand admixture comprising a salt-crystal, aprotein or a combination thereof, with a sand that comprises quartzsand, zircon sand, olivine sand, fireclay sand or a combination thereof,prepared according to the method of claim
 15. 20. A procedure forlayered composition of a metal casting mould, comprising the steps of:a) mixing solid particles of a bonding agent comprising a salt-crystal,a protein or a combination thereof, with a sand that comprises quartzsand, zircon sand, olivine sand, fireclay sand or a combination thereofto form a bonding agent/sand admixture, b) applying a thin layer of thebonding agent/sand admixture to an assembly field of an assemblyplatform; c) selectively applying water, in a sufficient dose so that itis capable of bonding particles of sand within a layer to each other, tounderlying sand particles that may be present, to the bonding agent/sandadmixture in required areas for reacting it with the bonding agent/sandadmixture; d) drying the water; e) lowering the assembly platform; andf) repeating at least steps (a)-(d) for applying an additional layer.21. The procedure of claim 20, further characterized by a step ofrecycling the sand from the resulting mould.